Quantum cascade lasers (QCL) are lasers in which the gain spectrum is typically broader than approximately 5% of the central wavelength of the laser. In typical configurations (such as Fabry-Perot configurations, exemplified by FIG. 2 without the grating 14), i.e., with one facet of the QCL 10 with high reflectivity coating and the exit facet with controlled reflectivity anti-reflection coating 12, the QCLs 10 will produce very high power output, approaching power greater than 4 W at a wavelength of approximately 4.6 micrometer. However, as shown in FIG. 1, this output occurs in a bandwidth of approximately 250 nm around the central wavelength.
Such broadband operation is acceptable when the precise wavelength or the bandwidth of the output is not critical, for example, for directional infrared countermeasures (DIRCM), targeting, beacon, and illumination applications. On the other hand, there are a significant number of very important applications, where the laser output must be narrow band, for example, less than 1 nm wide, and tunable over some wavelength region. These applications include spectroscopy and sensors for detection of pollutants, toxic gases and explosives. To obtain a “single frequency” output from a broadband gain spectrum laser such as a QCL, a wavelength dispersive element needs to be introduced within the laser cavity so that only one selected wavelength can resonate. Such dispersive elements include diffraction gratings 14 (FIGS. 2 and 3), prisms 18 (FIG. 4) and tunable or otherwise narrow band filters 20 (FIG. 5).
A key feature of all of these schemes is that mechanical motion is required to tune the wavelength of the laser since the wavelength selection is dependent on the angle as shown in the FIGS. 2-5. All the techniques shown in these figures permit tuning of the laser wavelength over the entire gain spectrum (as long as the round trip optical gain exceeds total cavity losses). But, the tuning is slow because of the mechanical motion of a discrete, dispersive element (grating, prism or filter) and not appropriate for applications calling for ruggedness, such as for sensors that would be deployed in the field, carried by personnel or mounted on vehicles. There are many applications that require very rapid tuning because there is a need to obtain a complete spectrum of the object under examination in a very short time. Such applications include studies of time dependent combustion dynamics and explosion dynamics, time dependent spectral changes during chemical and biological reactions, rapid examination of an improvised explosive device in standoff detection mode and tracking the release of toxic gases.
There is yet another way of obtaining narrow linewidth output from an otherwise broadband QCL. This is the use of distributed feedback grating, which is embedded within the gain structure of the laser. Such lasers are useful because they are simple to fabricate and are rugged. However, tunability is quite limited around the design wavelength of the distributed Bragg grating. Typical tuning range for distributed feedback lasers (DFB) is limited to approximately 5 cm−1 around the design wavelength of the grating. This is but a small fraction of the gain spectrum width of the QCL. The tuning can be carried out either by varying the QCL drive current or by changing the temperature of the QCL. In either case, no mechanical motion is required. The thermal tuning is slow while the electrical current driven tuning can be relatively fast. However, for obtaining broadband tuning, DFB lasers are inappropriate.
For the foregoing reasons there is a need for rugged, rapid broadband tuning of quantum cascade lasers.